Very high power densities made available by lasers have made it possible to observe the nonlinear behavior of optical media, such as crystals. Frequency doubling is an example of such nonlinear behavior. Frequency doubling is a specific example of what is known as the sum-frequency generation process, and occurs when an incident radiation of frequency v, on propagating through some crystalline material, emerges as radiation consists of a mixture of two frequencies, the original frequency v and a new frequency 2 v.
The inverse of the aforementioned sum-frequency process is the optical parametric process, wherein incident radiation having a frequency v, on propagating through a nonlinear medium is converted into two lower frequency (higher wavelength) waves, which are of essentially variable frequency.
Optical parametric systems can be used to generate secondary (parametrically generated) radiation from a monochromatic coherent (incident) primary radiation by means of parametric interaction with an optically nonlinear medium. The secondary radiation has two components, one of which typically has a shorter wavelength than the other, although it is possible that both of the two components can have the same wavelength. The condition is known as "degeneracy." The wavelengths of both secondary radiation components are longer than that of the primary radiation. The wavelength of a secondary radiation component can be freely selected and can be adjusted by a suitable arrangement, typically by rotation of the optically nonlinear medium within the optical resonator of the parametric system. The wavelength of the other component will be determined based on energy conservation. The optical parametric system can be used in combination with a coherent source of primary optical radiation, for example a laser, to provide a source of optical radiation whose wavelength can be selected more or less as desired within a desired frequency range.
In a parametric oscillator, the relationship between the "pump" frequency (f.sub.p) of the incident radiation and the "signal" (f.sub.s) and "idler" (f.sub.i) parametrically generated component frequencies is given as: f.sub.p =f.sub.s +f.sub.i
As the OPO is tuned away from "degeneracy", wherein the signal and idler frequencies are equal, the signal and idler wavelengths change, with the former (signal wavelength) decreasing and the latter (idler wavelength) increasing. Further, as the OPO is tuned well away from degeneracy, the signal and idler wavelengths become well separated, a consequence of which is that their indices of refraction will differ.
Usually, crystals are used as the optically nonlinear medium.
Examples of crystals exhibiting the desired nonlinear effects are KDP, LiNbO.sub.3, Ba2Na(NbO.sub.3).sub.5 or LiO.sub.3. Since these nonlinear media exhibit relatively weak parametric interaction, it is known to locate the nonlinear medium within an optical resonator formed by mirrors, so that the radiation passes repeatedly through the medium. However, this creates a substantial problem with respect to coupling radiation into and out of the resonator. Hence, it has been known to employ mirrors, defining the resonator, which are highly transmissive for the primary radiation and highly reflective for the secondary radiation. This requirement is difficult to meet, when the parametric arrangement is intended to be continuously tunable regarding the wavelength of the secondary radiation, since the mirrors have to be highly reflective at a region close to the wavelength of the primary radiation, and the range of high reflectivity should be wide over a wide range of wavelengths. In practice, the reflectivity should be on the order of 80 to 95%. Even the most highly developed dichroic mirrors, made of multiple dielectric layers, only partially meet this requirement, and then only at great cost.
U.S. Pat. No. 4,639,923 describes an optical parametric oscillator (OPO) using a urea crystal. This crystal makes the OPO broadly tunable, so that the entire spectral range from the ultraviolet to the near infrared is accessible. Further, the urea OPO is angle-tunable, and has a high efficiency.
U.S. Pat. No. 4,085,335 describes an optical parametric device wherein the coupling means, or arrangement in the parametric system, includes a dichroic mirror located in the path of the optical resonator, which dichroic mirror is arranged at an inclination with respect to the path of radiation within the resonator. The mirror has a high reflectivity for the primary radiation and a high transmissivity for the secondary radiation.
U.S. Pat. No. 4,180,751 describes a mode-locked optical parametric oscillator apparatus wherein the OPO cavity length is substantially smaller than the pump laser cavity length and in which the oscillator mirrors are singly resonant at either the signal or idler (pulse) frequencies. This configuration is intended to generate non-resonated OPO pulses which replicate mode-locked pump pulses, and both sets of pulses coupled to the resonated OPO pulse over a relatively wide tuning range without adversely affecting OPO operation.
Thus, it is seen that an OPO may be used to generate secondary radiation having a wavelength significantly offset from the primary radiation. In many instances, the weaker component (f.sub.i) of the secondary radiation is shunted, in that it is not of interest and is not independently tunable with respect to the stronger (f.sub.s) component.
In commonly-owned, copending U.S. patent application Ser. No. 544,497 entitled "Stacked Optical Parametric Oscillator" filed on Jun. 27, 1990, there is disclosed a stacked OPO wherein two or more optically nonlinear media, such as crystals, are coaxially disposed in a single resonator. Incident radiation is coupled into the resonator, and causes parametric oscillations of the two crystals. The two crystals are independently tuned, such as by angular orientation, to produce distinct components of secondary radiation. A first one of the crystals is disposed nearer to the source of incident radiation, and a second one of the crystals is disposed nearer to the output coupler of the resonator. This causes the first crystal to experience a greater effective gain. Furthermore, the secondary radiation from the first crystal will tend to dominate and "seed" the secondary radiation from the second crystal, when their bandwidths are narrowly separated. The dominance of the first crystal is controlled in various ways: 1) by shortening the length of the first crystal; 2) by differential bevelling of the first crystal with respect to the optical axis of the resonator; or 3) by detuning the output coupling mirror of the resonator with respect to the output of the first crystal. Either method effectively balances the effective gain of the two crystals so that two, independently tunable and efficient signal frequencies can be achieved. Seeding the OPO stack is also disclosed. Alternate techniques of seeding include the use of a tunable diode laser, a second low power OPO and a second OPO using a Faraday Anomalous Dispersion Optical Filter (FADOF). Techniques for angle tuning the OPO stack and compensating for walkoff are disclosed.
One of the inherent limitations with an OPO is that its output tends to be broader than the pump laser line. This broad output has limited the usefulness of an OPO.
In order to line narrow the OPO output, gratings have, in the past, been placed in the OPO cavity or used to form one of the end mirrors of the optical cavity. Although these arrangements serve to line narrow the OPO output, the additional optical elements in the OPO reduces the overall OPO efficiency, thus limiting its usefulness in low power applications. Furthermore, if the OPO achieves high power operation, the grating can be destroyed.
It is therefore an object of the present invention to provide an OPO having a new and improved arrangement for line narrowing the output.
It is another object of this invention to provide an OPO having a line narrowed output and wherein the amount of the line narrowing can be selectively changed.
It is a further object of this invention to provide an OPO having a variable line narrowed output.